What an idea!

...
That said, it could be argued that if people haven't upgraded yet (we'll
have given them 10 major releases to update to since 4.0), they likely
won't. That said, it could be argued that we should "brick" their
Firefox installation to keep them and the web safe.

It is possible that these users have installed add-ons that don't
qualify for DTC, though. An add-on needs to support at least Firefox 4
to qualify, and there were many add-ons that were never adapted to work
on 4. It might look disingenuous if we tell people their add-ons will
automatically work and then they don't.
We're looking for alternatives to popular pre-Firefox 4 add-ons to try
to move as many users as possible to them before the EOL.
- Jorge